It is known that electric power demand varies with a time period in a day. Electric power companies prepare facilities capable of dealing with a time period of peak electric power demand in order to accommodate the electric power demand during such a time period. Then, during a time period of low electric power demand, the electric power companies stop the operation of such facilities partially to change the power generation in accordance with the electric power demand. However, leveling of the power generation has been demanded to improve the operation efficiency of the facilities.
Patent Document 1 describes a system configured to provide a commodity to a user who charges a battery during a time period of low electric power demand and uses power stored in the battery during a time period of high electric power demand.
Patent Document 2 describes a system providing a resident of an apartment house with a power rate for using power in a battery stored during a time period of low electric power demand, to which a lower power rate is set by an electric power company, during a time period of high electric power demand, along with a rate for using the battery.